Volume 17 Preprint 46
MODELING OF ADSORPTION ISOTHERM FOR METHANOL LEAF EXTRACT OF Manihot esculentum AS GREEN CORROSION INHIBITOR OF CORROSION OF MILD STEEL IN HCl MEDIUM
Sylvester. O. Adejo1&2*, M. M. Ekwenchi2, John U. Ahile1, Joseph. A. Gbertyo1, Benjamin Ishua1, Andrew Akombor1
Keywords: Adsorption, Mild steel, Manihot esculentum, Adejo-Ekwenchi isotherm, physisorption
The best fit adsorption isotherm model for methanol leaf extract of Manihot esculentum as green corrosion inhibitor of corrosion of mild steel in 2 M HCl medium was determined with the help of the Adejo-Ekwenchi adsorption isotherm. The corrosion inhibition study was carried out using the weight loss method at the temperature range of 301 K to 313 K. The inhibition efficiency, %IE, of the extract was found to increase with increase in both concentration and temperature, suggestive chemisorption. Going by the values of %IE, R2 and Î”Gads this adsorption process would have been thought to be chemisorption and well fitted into Langmuir, Freundlich and Temkin isotherm models. However, from the information obtained through the Adejo-Ekwenchi isotherm model the adsorption process was unambiguously resolved to be physisorption and best fitted into the Langmuir isotherm model.
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MODELING OF ADSORPTION ISOTHERM FOR METHANOL LEAF EXTRACT OF
Manihot esculentum AS GREEN CORROSION INHIBITOR OF CORROSION OF
MILD STEEL IN HCl MEDIUM
Sylvester. O. Adejo1&2*, M. M. Ekwenchi2, John U. Ahile1, Joseph. A.
Gbertyo1, Benjamin Ishua1, Andrew Akombor1
Physical Chemistry Unit, Department of Chemistry, Benue State University,
Department of Pure and Applied Chemistry, University of Jos, Jos-Nigeria
Author of Correspondence: firstname.lastname@example.org +2348054557652
The best fit adsorption isotherm model for methanol leaf extract of Manihot
esculentum as green corrosion inhibitor of corrosion of mild steel in 2 M HCl
medium was determined with the help of the Adejo-Ekwenchi adsorption
isotherm. The corrosion inhibition study was carried out using the weight loss
method at the temperature range of 301 K to 313 K. The inhibition efficiency,
%IE, of the extract was found to increase with increase in both concentration and
temperature, suggestive chemisorption. Going by the values of %IE, R 2 and ΔGads
this adsorption process would have been thought to be chemisorption and well
fitted into Langmuir, Freundlich and Temkin isotherm models. However, from the
information obtained through the Adejo-Ekwenchi isotherm model the
adsorption process was unambiguously resolved to be physisorption and best
fitted into the Langmuir isotherm model.
Keywords: Adsorption, Mild steel, Manihot esculentum, Adejo-Ekwenchi isotherm,
Adsorption has been adjudged to be an effective process for a wide variety of
[1, 2]. Empirically, the process of adsorption can be described in
terms of amount adsorbed as a function of pressure (or concentration) and
temperature [3, 4]. For the adsorption process, the relationship between the
amount of the adsorbed substance to its bulk concentration at a particular
temperature and pH is known as adsorption isotherm, and many adsorption
isotherms have been proposed with their characteristic parameters. The
importance of the parameters of adsorption isotherms lay in the fact that they
useful for the deduction of the mechanism of adsorption as well as for design
purposes [1, 5-9].
Several error analysis methods such as coefficient of determination, the sum of
the errors, a hybrid error function, Marquardt‟s per cent standard, the average
error and sum of absolute error, have been employed in the determination of the
best-fitting isotherm for any adsorption process . Among these, the method
of least squares with linearly transformed isotherm equations has be acclaimed
as the most widely used for finding the best fit of adsorption isotherm . Also
the Gibb‟s free energy, ΔGads, value is usually employed to ascertain whether the
adsorption process is physisorption or chemisorption. But, Ho and Co-workers
 have noted the inadequacy of usage of coefficient of determination, R 2, for
the determination of best isotherm fit thus “the transformation of non-linear
isotherm equations to linear forms implicitly alters their error structure and may
also violate the error variance and normality assumptions of standard least
squares”. Again, it has been stated by many authors that value of ΔGads below -20
kJ/mol is an indication of physisorption, while above
-40 kJ/mol is ascribed to
be chemisorption. However, Popova and his research team  argued that the
use of ΔGads as a sole criterion for such distinction will be difficult, especially in
the case where charged species are adsorbed due to possibility of coulombic
interaction between charged species; it can increase the ΔGads value, even if no
new chemical bonds are formed. Evidently, therefore, there exist ambiguities in
the usage of R2 and ΔGads values as sole criteria in the determination of isotherm
best fit of adsorption process. Unambiguously, it has been shown that the AdejoEkwenchi isotherm can be used to eliminate the observed inadequacies in
adsorption characterisation process, especially in corrosion inhibition using
plant extracts [12- 14, 13].
In this present paper we unambiguously resolved the adsorption isotherm best
fit for methanol leaf extract of Manihot esculentum as green corrosion inhibitor
of corrosion of mild steel in 2 M HCl medium.
Leaves of Manihot esculentum were collected and rinsed with distilled water,
then shade-dried and pounded into powder. 30 g of the powder was soaked in
300 mL of 98% methanol (BDH Chemicals Ltd, England) in a 500 mL volumetric
flask for 48 hours, with occasional shaking. The extract obtained therefrom was
filtered using glass wool and the methanol was slowly evaporated using
thermostated water bath (Clifton: Nickel-Electro Ltd, England) at 321 K for
reason given elsewhere . The residue obtained was preserved in a desiccator.
Stock solutions of
0.1, 0.2, 0.3, 0.4 and 0.5 g/dm3 concentrations were
prepared in 2 M HCl (M & B).
Coupons (2 cm x 2 cm x 0.13 cm each) were prepared as reported before 
from a sample of mild steel of composition (%W): Fe(98.84), Mn(0.56), P(0.04),
C(0.27), Si(0.25) and S(0.04). The coupon surfaces were thoroughly polished to
mirror finishing using different grades of emery paper, degreased by washing
with ethanol (BDH), dried in acetone, and preserved in a desiccator for use.
The coupons, degreased in acetone (99% BDH Chemicals Ltd, England), were
carefully weighed using ae Adam AFP electronic weighing balance (d ±0.0001g)
and immersed in 50 mL of the acid solutions, without and with various
concentrations of the inhibitor at the temperature range of 301 K to 313 K in the
water bath for 8 hours. At the retrieval of each coupon, the reaction was
terminated by dipping into saturated solution of ammonium acetate (97%
Labtech Chemicals Limited) as outlined by Orubite-Okorosaye and Oforka 
and the coupon stored in the desiccator until ambient temperature was attained
and reweighed. An average of triplicate values was taken for each measurement.
The %IE and surface coverage ( ) were calculated through equations (1) and (2),
are weight losses in absence and present of inhibitor,
Results and Discussion
Table 1 shows the variation of %IE of the methanol leaf extract of Manihot
esculentum as eco-friendly corrosion inhibitor of the corrosion of mild steel
sample in 2 M HCl with the extract concentration and temperature. The Table
clearly shows that the %IE increases with increase in both concentration of the
extract and rise in temperature. The increase in the %IE with rise in temperature
is suggestive of chemisorption [22, 23].
Table 1. Variation of Inhibition Efficiency (%IE) with Concentration
and Temperature of Leaf Extract of Manihot esculentum
301 K 305 K
The inhibitive action of organic compounds has been ascribed to the formation
of surface layers and films on the metal surface, thereby reducing the
accessibility of the corrodant to the metal surface. Adsorption isotherms have
been used to characterise this inhibitive action.
The most frequently used of
such isotherms are the Langmuir, Freundlich, Frumkin, Temkin, Flory-Huggins,
Sips, Bockris-Swinkels and the El-Awady thermodynamic-kinetic model and the
recent proposed Adejo-Ekwenchi isotherm [1, 12, 24, 25]. The linearised forms
of the various equations are given below.
= log K + xlog [1 - θ]
= log K’ + ylog [C]
Adejo – Ekwenchi
The fitness adsorption data to an adsorption isotherm has been determined by
many authors through the use of the value of regression coefficient, R2, from the
plot of the surface coverage against the bulk concentration of the adsorbate
(extract). Going by this it can be asserted that the data for this extract fitted into
the Langmuir, Freundlich, Temkin, El-Awady and Adejo-Ekwenchi isotherms
(Table 2), not fitting other popular two-parameter isotherm like Flory-Huggins,
Frumkin and El-Awady.
The equilibrium constant, Kads, values of the isotherms (Table 2) are all positive,
implicitly, indicative of favourable adsorption in all the isotherms . The Sshaped curve of the plot of %IE against the logarithm of concentration in Figure 1
(at 301 K) further attests to the fact that an adsorbed layer was actually formed
on the metal surface by the extract [8, 26].
Table 2. Various Adsorption Isotherm Parameters for Adsorption of Leaf
Extract of Manihot esculentum onto the mild steel surface
Figure 1. Relationship between Inhibition Efficiency and
logarithm of Concentration at 301K for the Extract of
Equations (11) and (12) are conventionally used to calculate ΔGads for an
C is the concentration of the extract and 55.5 is concentration of water
expressed in moles (i.e. ~ 1000 g/dm3) [12, 27]. The values of ΔGads evaluated
through the equation (11) are presented in Table 3.
The slope and intercept of ideal Langmuir isotherm plot should be unity and
zero, respectively [28, 29]. The slopes of the Langmuir plot obtained are close to
unity and intercepts almost equal to zero, with good R2 values, indicative of
suitability of the Langmuir isotherm model to the adsorption behaviour of this
extract. Moreso, the values of ΔGads obtained through this isotherm are also in
good agreement with those evaluated through the conventional method as
shown in Table 3. Langmuir isotherm is applicable to both physisorption and
.Table 3. Values of ΔGads obtained through equilibrium constant
- ΔGads (kJ/mol)
The parameter n of the Freundlich isotherm relates to the adsorption intensity,
and it varies with the heterogeneity of the material  and it has a typical value
is 0.6 , of which no value is close (Table 2), except at 301 K. And the values
of ΔGads obtained through this isotherm model are also not comparable to those
of the conventional method. Therefore, the adsorption process of leaf extract of
Manihot esculentum onto the metal sample in HCl cannot be modelled by the
Freundlich isotherm, in spite of the seemly good values of R2.
The Temkin isotherm mostly is a feature of chemisorption  and an indication
of interaction of uncharged molecules on a heterogeneous surface . The
negative values of parameter α are clear indication of the existence of repulsive
interaction among the molecules of the adsorbed layer. The values of R 2 are
good, but those of ΔGads show no favourable adsorption by this extract onto the
metal surface, a complete contrast to the observation above, and hence cannot
be modelled by Temkin isotherm.
The Adejo-Ekwenchi isotherm is centred on the fact that for an adsorption
process, the amount of adsorbate uptake from bulk concentration has inverse
relationship with the difference between the total available surface on the
adsorbent surface and the fraction that is covered by the adsorbate at a given
temperature, prior to the attainment of maximum value of surface coverage .
Decrease in the value of b of the isotherm with rise in temperature signifies
physisorption, while increase or fairly constant value indicates chemisorption
[12, 13, 33]. From Table 2 it is obvious that the adsorption of this plant extract
onto the metal surface is physisorption as b decreases with rise in temperature.
Supportively, the values of ΔGads obtained through this isotherm are close to
those of Langmuir isotherm and the conventional method.
Going by %IE, R2 and ΔGads values, the adsorption process of methanol leaf
extract of Manihot esculentum as an inhibitor for the corrosion of mild steel in 2
M HCl which would have been thought to be chemisorption and the data fitted
into Langmuir, Freundlich, and Temkin isotherm models, was resolved through
Adejo-Ekwenchi isotherm to be physisorption and best fitted only to the
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